The present invention relates to a data demodulation apparatus in use for an image filing system for recording and reproducing image data by using an optical disc.
To store a large number of documents as produced in offices, etc., the image filing system using the optical disc has been widely used. In this system, an original such as a document is optically scanned in a two dimensional manner. The optical image data thus obtained is converted into the corresponding electrical image data. Such data is recorded into an optical disc. If the document recorded is needed later, it is retrieved and reproduced, and printed out on a record paper or displayed by a CRT display device. For obtaining a high density recording of the image data, the electrical image data is 2-7 code converted (modulated) before recording. To reproduce the image data, it is converted by the process reverse to that of the 2-7 code conversion (demodulation).
For the demodulation, the clock pulse which has been used for modulating the data, is required for the reproduction pulse. When the rotating speed of the optical disc is temporarily changed, the phase of the code data also changes. In demodulating such code data, since the reproduction pulse has fixed phase and frequency, an improper demodulation of the code data is performed. To cope with this, the reproduction pulse is phase synchronized with the code data. The reproduction pulse is first synchronized in phase and frequency with the clock pulse, and then in phase with the code data. In the prior art system, once the frequency of the reproduction pulse is equal to that of the clock pulse, the phase of the reproduction pulse is controlled so as to coincide with that of the clock pulse irrespective of the frequency of the reproduction pulse. When the PLL circuit suffers from noise, the frequency of the reproduction pulse is shifted from its correct one. However, the prior art system can not correct such a shift. This often results in incorrect demodulation of the code data.